In many technical processes, hydrogen chloride is obtained as a by-product in the form of a more or less strongly diluted, aqueous hydrochloric acid, for which there is merely a limited range of technical usefulness.
In order to be able to utilize the hydrogen chloride present in the form of such a dilute, aqueous hydrochloric acid as a chemical raw material, for example for the manufacture of vinyl chloride, ethyl chloride, chloroprene, or chlorosulfonic acid, this compound must be isolated as an anhydrous gas.
A maximally 20.2% strength hydrochloric acid can be obtained from dilute, aqueous hydrochloric acid solutions by distillation under normal pressure; this hydrochloric acid forms in this concentration an azeotrope boiling at 109.degree. C. Although the composition of the azeotrope can be altered by varying the pressure, so that finally also gaseous hydrogen chloride could be produced from a dilute hydrochloric acid solution by combining two columns operated under different pressures, such a procedure is unfeasible on a large technical scale, because large quantities of water must be vaporized with the expenditure of a large amount of energy. Likewise uneconomical are methods wherein the hydrogen chloride is displaced from the aqueous phase by salts, such as calcium chloride or magnesium chloride, or wherein the water is engaged, for example, by concentrated sulfuric acid, because these additives must be regenerated in a subsequent stage under evaporation of the water.
It has, therefore, been suggested to extract aqueous hydrochloric acid solutions with special solvents, e.g. with pentanols, and to obtain the hydrogen chloride thereafter as concentrated hydrochloric acid (E. D. Crittenden, A. N. Hixson, Ind. Eng. Chem. 46 [1954] : 265-274). Since the pentanol phase absorbs considerable amounts of water in addition to the hydrogen chloride, however, only minor amounts of anhydrous hydrogen chloride are obtained by distillation; the larger portion of the hydrogen chloride again appears as an azeotrope with water.
The distribution coefficient of hydrogen chloride between isoamyl alcohol and water ranges, in dependence on the concentration, between 0.13 and 0.32 with 5% and 15% of hydrogen chloride in the water, respectively; in other words, only the smaller portion of the hydrogen chloride enters the extract phase with identical amounts of extract and raffinate.
Suitable extractants with far more favorable distribution coefficients are longer-chain amines, the hydrochlorides of which, due to the hydrophobia of the alkyl chains, are no longer soluble in water, or exhibit an only very low solubility therein. Such amines, e.g. methyldioctylamine (C.A. 1964 : 10102h), trilaurylamine (C.A. 61 : 10092 C), trioctylamine (C.A. 71 : 85106 C), N-octylaniline (C.A. 68 : 131337 C), hexadecylamine (C.A. 79 : 10397 e), are capable, optionally in the presence of further inert organic solvents, such as benzene, xylene, or aliphatic hydrocarbons in the benzine range, of almost quantitatively extracting hydrogen chloride from aqueous solutions. Therefore, by means of such an extraction with amines, a process is available which is readily usable from a technical viewpoint, for almost quantitatively removing hydrogen chloride from its aqueous solution and thus to obtain a practically acid-free, aqueous raffinate. However, no simple technical method has been known heretofore in order to obtain hydrogen chloride from the thus-produced extracts.
It is known that aniline can likewise be utilized for extracting a portion of the hydrogen chloride from an aqueous hydrochloric acid solution (N. V. Sidgwick, P. Pickford, and B. H. Wilsdon, J. Chem. Soc. 99 [1911] : 1122), thus producing the hydrochloride of aniline. It is known that aniline hydrochloride decomposes, at its boiling point of 245.degree. C. under normal pressure, completely into its components aniline and hydrogen chloride. However, it is impossible to separate the hydrogen chloride from the vapor mixture with simple means, for example by condensation of the aniline contained therein; this is so, because during the cooling of the vapor mixture, a complete reformation of the aniline hydrochloride takes place.
Due to the lack of a usable method for separating the hydrochlorides of amines into their components, the methods for the extraction of hydrogen chloride with amines have heretofore been limited to the regeneration of the amines, by treating the thus-obtained ammonium chloride with alkalis, wherein the amine was liberated but the hydrogen chloride was bound as the salt (for example E. L. Smith, J. E. Page, J. Soc. Chem. Ind. 67 (1948) : 48.
In summation, it is to be noted that up to the present time, there has not been known any method, which can be conducted with an economically feasible expenditure, for obtaining gaseous hydrogen chloride with high yields from aqueous, particularly diluted hydrochloric acid solutions.